1. Field of the Art
This invention relates to a projection lens system suitable for use on projectors for projection of CRT (cathode ray tube) picture images on a large viewing screen.
2. Description of the Prior Art
Projectors are generally arranged to project picture images of projection tubes of three colors of red (R), Green (G) and blue (B) through a projection lens system which is located forward of the projection tubes, thereby to project tricolor picture images on a large viewing screen located forward of the lens system. Heretofore, it has been the general practice for the projection lens systems of this sort to employ glass or plastic lenses, without paying much attention to the necessity for positive correction means for chromatic aberration because of the narrow luminous spectral width of the fluorescent material of the projection tube which is akin to the spectral width of monochromatic light.
However, in order to meet the recent demands for higher picture quality, the projection lens systems especially for high-definition TV or for large screen projectors are required to be able to make corrections for chromatic aberration to a sufficient degree in addition to high resolution power. For the purpose of meeting these requirements, it is conceivable to construct a projection lens system which has all of its lens elements constituted by plastic lenses. In this connection, the correction of monochromatic aberration is possible by the use of plastic lenses which can be easily formed in large diameter and aspheric shape by injection molding. Under the current circumstances, however, the plastic lenses have a limited freedom in selection of material and, in addition to insufficient correction of chromatic aberration, involve an objectionable degree of temperature dependent shifts of the image point due to large temperature and linear expansion coefficients of the refractive index of plastic material.
On the other hand, a projection lens system which is constituted by glass lenses alone can easily make corrections for the chromatic aberration and temperature-dependent shift of the image point, but normally there have to be provided at least six to seven glass lenses, resulting in increased production cost and weight.
In an attempt to overcome these problems, there have been developed the so-called hybrid type projection lens systems which employ a combination of plastic and glass lenses. The systems of this type are markedly improved in chromatic aberration and temperature dependent shift of the image point as compared with the systems which are composed of glass or plastic lenses alone, but still need further improvements in these and other respects to meet the recent demands for higher picture quality.